A pair of binoculars provides three-dimensional magnified images using two parallel mirror-symmetric optical units. In the conventional binoculars, each optical unit comprises an objective lens group, a prism lens group, and an ocular lens group. The objective lens group gathers incident light from an object. The prism lens group typically comprising two prisms is configured to change and fold the path of light and provide an upright image. The ocular lens group magnifies the image formed by the objective lens group and the prism lens group. As the object distance is changed, the focus of the binoculars should be adjusted unless the optical units have a long depth of field. In the conventional binoculars, the distance between the objective lens group and the ocular lens group is configured to be adjusted by macroscopic mechanical lens movement in order to change the focus of the binoculars. However, the repeated macroscopic mechanical lens movement can cause wear and tear and misalignment problems. Also, the moving mechanism and multiple prisms can add significant weight to the binoculars which may adversely affect to their portability.
Binoculars having zoom function have not been used widely because they require complicated macroscopic mechanical lens movement and precise collimation of optical elements. Also, the image quality of the conventional binoculars having zoom function is inferior to that of fixed magnification binoculars because it is very difficult to correct the optical aberration of the binoculars throughout an entire zoom range. Therefore, the conventional binoculars having zoom function tend to be bulky, fragile, and expensive with inferior image quality, which are undesirable especially for outdoor use. Therefore, most conventional binoculars provide a fixed power.
The advanced binoculars should be light weighted and ragged with high image quality. Also, they should be able to provide focusing and/or zoom functions for versatile use.